Beam Scanning Based on Two-Dimensional Polygon Scanner for Display and Other Applications

ABSTRACT

Scanning beam systems based on a two-dimensional polygon scanner with different reflective polygon facets tilted at different tilt facet angles to use rotations of the polygon scanner to scan optical beams both horizontally and vertically on a surface which can be a display screen or a printing surface.

BACKGROUND

This application relates to techniques, apparatus and systems forscanning beams by using polygon scanners.

Image and video displays can be designed to scan one or more opticalbeams on a screen. Polygon scanners can be used in such display systems.For example, some polygon-based display systems use one or moremodulated optical beams that carry image information to produce imageson screens by using a polygon scanner to scan an optical beamhorizontally and a vertical scanner to scan the optical beam vertically.Such scanner systems can be used in systems other than display systems.

SUMMARY

This application provides techniques, apparatus and designs for scanningbeam systems based on a two-dimensional polygon scanner with differentreflective polygon facets tilted at different tilt facet angles to userotations of the polygon scanner to scan optical beams horizontallywhile stepping them vertically on a surface such as a display screen ora printing or imaging surface.

In one aspect, a scanning beam system is provided to include an opticalmodule operable to produce a plurality of optical beams modulated tocarry images to scan over a surface to produce the images on the surfacebased on a two dimensional scanning pattern. The optical module includesa polygon scanner positioned in optical paths of the optical beams. Thepolygon scanner includes a rotation axis around which the polygonscanner rotates to scan the optical beams horizontally on the surface, aplurality of polygon facets that are sized to simultaneously receive theoptical beams and comprise a plurality of reflective polygon facets aretilted with respect to the rotation axis at different facet tilt angles,respectively, to scan the optical beams horizontally at differentvertical positions on the surface, respectively. This optical modulealso includes a vertical adjuster placed in the optical paths of theoptical beams to control and adjust vertical positions of the opticalbeams on the surface; and a control unit that controls the verticaladjuster at a fixed position to place the optical beams at respectivefixed vertical positions on the surface when the polygons scannerhorizontally scans the optical beams to produce parallel horizontallines on the surface.

In another aspect, a scanning beam system includes an optical moduleoperable to produce a plurality of optical beams modulated to carryimages to scan over a surface to produce the images on the surface. Thisoptical module includes a polygon scanner positioned in optical paths ofthe optical beams and the polygon includes a rotation axis around whichthe polygon scanner rotates to scan the optical beams horizontally onthe surface, and a plurality of polygon facets that are sized tosimultaneously receive the optical beams and reflective to light of theoptical beams. The polygon facets are tilted with respect to therotation axis at different tilt angles, respectively, to scan theoptical beams horizontally at different vertical positions on thesurface, respectively. This optical module also includes a verticaladjuster placed in the optical paths of the optical beams and to controland adjust vertical positions of the optical beams on the surface, and ascanning control mechanism to synchronize the vertical adjuster to thepolygon scanner to adjust vertical positions of the optical beams on thesurface to spatially interlace one frame of a sequential sets ofsimultaneous horizontal scanning lines on the surface produced by thepolygon facets, respectively, one set per polygon facet, in one fullrotation of the polygon scanner with a subsequent frame of a sequentialsets of simultaneous horizontal scanning lines on the surface producedby the polygon facets, respectively, one set per polygon facet, in animmediate subsequent full rotation of the polygon scanner.

In yet another aspect, a method for scanning optical beams in a scanningbeam system includes producing a plurality of optical beams modulated tocarry images to scan over a surface to produce the images on the surfaceand using a polygon scanner in optical paths of the optical beams toscan the optical beams horizontally on the surface. The polygon scannerincludes a rotation axis around which the polygon scanner rotates andpolygon facets that are sized to simultaneously receive the opticalbeams and reflective to light of the optical beams. The polygon facetsare tilted with respect to the rotation axis at different tilt angles,respectively, to scan the optical beams horizontally at differentvertical positions on the surface, respectively, to produce one frame ofa sequential sets of simultaneous horizontal scanning lines on thesurface produced by the polygon facets, respectively, one set perpolygon facet, in one full rotation of the polygon scanner. This methodincludes holding a vertical position of each optical beam on at a fixedposition when each optical beam is being horizontally scanned on thesurface and the light of each optical beam is projected onto thesurface, without scanning each optical beam along the verticaldirection.

These and other examples and implementations are described in detail inthe drawings, the detailed description, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a scanning beam display system based on a combination of atwo-dimensional polygon scanner with differently tiled facets and avertical adjuster for a display screen that can either a passive screenor a light-emitting screen under optical excitation.

FIG. 1B illustrates an example of the scanning by the two-dimensionalpolygon scanner and the vertical adjuster in FIG. 1 to interlace twofields, field 1 and field 2, into a full frame.

FIG. 1C shows an example scanning laser display system having afluorescent screen made of laser-excitable fluorescent materials (e.g.,phosphors) emitting colored lights under excitation of a scanning laserbeam that carries the image information to be displayed.

FIGS. 2A and 2B show one example screen structure and the structure ofcolor pixels on the screen in FIG. 1C.

FIG. 3 shows an example implementation of the laser module in FIG. 1Chaving multiple lasers that direct multiple laser beams on the screen.

FIG. 4A shows an example of a two-dimensional polygon scanner withidentical polygon facets with different tiled facet angles.

FIG. 4B illustrates operation of a two-dimensional polygon scanner.

FIG. 5 illustrates an example of blanking periods in a 2-dimensionalscanning pattern on the screen by using the polygon scanner in FIGS. 4Aand 4B and a vertical adjuster that shifts the vertical positions of thebeam incident to the polygon scanner.

FIGS. 6A and 6B show additional examples of two-dimensional polygonscanners with a different facet designated for create a blanking timeduring the polygon scanning.

FIGS. 7A, 7B, 8A and 8B show examples of a scanning system with atwo-dimensional polygon scanner in a bottom or upper feed arrangement.

FIGS. 9A, 9B, 10A and 10B show examples of a scanning system with atwo-dimensional polygon scanner in a side feed arrangement.

FIG. 11 shows a stacking operation of the vertical adjuster based onhorizontal scanning and vertical stepping of the two-dimensional polygonscanner in a beam scanning system.

DETAILED DESCRIPTION

Various display systems based on scanning one or more optical beams on ascreen use a combination of a horizontal polygon scanner and a verticalscanner to produce a desired raster scanning pattern on the screen toproduce images. For example, in some implementations, the horizontalpolygon scanner can be used to scan an optical beam only horizontallywithout performing vertical scanning functions and the vertical scannercan be used to scan of the beam on the screen without performing anyhorizontal scanning. Such horizontal and vertical scans are usuallysynchronized to each other to perform the scanning simultaneously.Hence, as the polygon scanner scans the beam horizontally, the verticalscanner simultaneously scans the beam vertically. As a result, eachscanning trace of the beam on the screen is a slanted line and is nothorizontal. In this combination of simultaneous horizontal scanning andvertical scanning, the vertical scanner is designed to have a sufficientvertical angular scanning range to cover all desired vertical positionson the screen, an acceptable linear range for the vertical scanning, anda sufficiently short response time for desired refresh rate and retracetime of the raster scanning. In high definition display systems, such as1080p HDTV systems, various technologies and designs for verticalscanners may be difficult to meet the scanning requirements due to thehigh scanning rates associated with high resolution nature of thedisplay, limited space for accommodating the optical path from thevertical scanner to the screen, and other factors.

The examples and implementations of scanning beam systems for displayand other applications in this patent application are based on atwo-dimensional polygon scanner with different reflective polygon facetstilted at different tilt facet angles to use rotations of the polygonscanner to scan optical beams horizontally without simultaneous verticalscanning to produce horizontal scan lines on the screen and adjustvertical positions of the optical beams during a blanking time whenthere is no light projected onto the screen by using different facets toperform the horizontal scanning. A vertical adjuster can be used incombination with the two-dimensional polygon scanner to provide anadditional vertical adjustment to the vertical positions of the beamsduring a blanking time when there is no light projected onto the screento increase the number of horizontal lines on the screen. This verticaladjuster can include a reflector to reflect each beam and an actuator tocontrol the orientation of the reflector to adjust the vertical positionof a beam on the screen. The vertical adjuster is operated to hold thevertical position of a beam at a fixed vertical position on the screenwhen the beam is being horizontally scanned on the screen. Hence, thevertical adjuster in such implementations does not perform theconventional vertical scanning due to the operation of thetwo-dimensional polygon scanner. Therefore, the present use of thetwo-dimensional polygon scanner can lessen the technical performanceparameters for the vertical adjuster in comparison with a verticalscanner and to allow a variety of beam deflection devices with adjustactuators to be used as the vertical adjuster, such as various1-dimensional beam scanners, reflectors coupled with step actuators andothers, to be used in scanning display systems based on thetwo-dimensional polygon scanner described in this patent application. Asa specific example, a beam deflector having a reflective mirror and asweeping or step galvanometer actuator engaged to the mirror may be usedto implement the vertical adjuster.

FIG. 1A shows illustrates an example of a scanning beam display systembased on a two-dimensional polygon scanner with different reflectivepolygon facets tilted at different tilt facet angles to produce parallelhorizontal lines at different vertical positions on the screen and avertical adjuster to adjust vertical positions of parallel horizontallines in one group to relative to vertical positions of parallelhorizontal lines in another group produced in time subsequent to theprior group on the screen. The vertical adjuster can be controlled toproduce an interlaced scanning pattern formed by the two or more groupsof the parallel horizontal lines or other scanning patterns. Thevertical and horizontal directions are used in this patent applicationto represent two orthogonal directions in general and are not intendedto represent any specific directions such as the vertical direction withrespect to the earth's gravity. This system includes a screen 1 on whichimages are displayed and a laser module 10 that produces and scans oneor more scanning optical beams 12 onto the screen 1. An optical beam 12is modulated to carry images, a sequence of laser pulses that carryimage data. The laser module 10 scans the one or more optical beams 12in a raster scan pattern to display the images on the screen 1.

The two-dimensional polygon scanner and the vertical adjuster areincluded as part of the scanning module inside the laser module 10. Oneor more lasers are included in the laser module to produce the one ormore optical beams 12. A scanning control module is provided to controlthe polygon scanner and the vertical adjuster. The polygon scanner ispositioned in optical paths of the one or more optical beams 12 andincludes a rotation axis along the vertical direction and the polygonscanner rotates around this rotation axis to scan the optical beams 12horizontally on the screen 1 along the horizontal scanning direction asshown. The polygon is designed to have multiple polygon facets that aresized to simultaneously receive the one or more optical beams 12directed from the one or more lasers. The polygon facets are reflectiveto light of the optical beams 12 and tilted with respect to the rotationaxis at different tilt angles, respectively, to scan the optical beamshorizontally at different vertical positions on the screen,respectively. The vertical adjuster is placed in the optical paths ofthe optical beams 12 to adjust vertical positions of the optical beamson the screen.

In operation, the polygon scanner rotates to scan the scanning beams.Each polygon facet receives, reflects and scans the one or more beams 12horizontally on the screen 1. The immediate next polygon facet is tiltedat a different tilt angle and thus receives, reflects and scans the sameone or more beams 12 horizontally at different vertical positions on thescreen 1. In systems with multiple optical beams 12, the differentoptical beams from one polygon facet are directed to different verticalpositions on the screen 1. As different polygon facets sequentially taketurns to perform the horizontal scanning of the one or more beams 12 asthe polygon scanner rotates, the vertical positions of the one or morebeams 12 on the screen 1 are stepped vertically at different positionsalong the vertical stepping direction without a conventional verticalscanner in other scanning systems. During the time when a facet scansthe one or more beams 12 on the screen 1, the vertical adjuster isoperated at a fixed orientation so that each beam 12 is being scannedonly along the horizontal direction without a simultaneous verticalscanning.

FIG. 1B illustrates one example for interlaced raster scanning for the2D polygon scanner and the vertical adjuster in FIG. 1A. Assume thereare M facets in the polygon and N optical beams 12. The tilt facetangles of the polygon facets can be designed to vertically divide thescreen into M vertical segments to project N parallel horizontal scanlines in each vertical segment. In some implementations, the linespacing between two adjacent lines of the N lines can be set to allowfor at least one horizontal scan line and this configuration can be usedto support interlaced scanning operations. As the polygon rotates,different facets direct and scan different vertical segments atdifferent times, one at a time. Hence, scanning by different polygonfacets in one full rotation of the polygon scanner produces a frame orfield of M×N horizontal scanning lines that are made of M sequentialsets of N simultaneous horizontal lines. This operation provides bothhorizontal scanning by each facet and vertical stepping by sequentiallychanging the polygon facets. Therefore, in one full rotation, thepolygon scanner produces one frame of a sequential sets of simultaneoushorizontal scanning lines on the screen produced by the polygon facets,respectively and each polygon facet produces one set of simultaneous andhorizontal scanning lines.

Notably, during each full rotation, the vertical adjuster is controlledat a fixed orientation. After completion of one full rotation of thepolygon and before the next full rotation of the polygon, the verticaladjuster is operated to adjust its orientation to change verticalpositions of the optical beams 12 on the screen 1 to spatially interlacehorizontal scanning lines in one frame produced in one full rotation ofthe polygon scanner with horizontal scanning lines of a subsequent frameproduced in an immediate subsequent full rotation of the polygonscanner. The vertical adjuster and the polygon scanner are synchronizedto each other to perform the above interlaced raster scanning. In theexample in FIG. 1B, each full frame image is formed by two frames orfields, Field 1 and Field 2, that are spatially interlaced and the linespacing between two adjacent lines produced by each facet is onehorizontal scan line to facilitate the interface operation. Hence, thevertical adjuster in this example, is operated to operate at twoorientations, one orientation for the Field 1 and another for the Field2, respectively. The rate for the vertical adjustment of the beamposition is only 2 per full frame.

In the system in FIG. 1A, the screen 1 and the laser module 10 can beimplemented in various configurations. For example, the screen 1 can bea passive screen that does not emit visible light and renders images byreflecting, diffusing or scattering visible light of the one or moreoptical beams 12 that carry images and the one or more optical beams 12are visible beams, e.g., red, green and blue beams.

For another example, the screen 1 in FIG. 1A can be a screen thatabsorbs light of the one or more optical beams 12 which may be UV orviolet light and emit visible light that renders the images carried bythe one or more optical beams. Such a system uses a screen withlight-emitting materials, such as phosphor and fluorescent materials, toemit light under optical excitation to produce images. Various examplesof screen designs with light-emitting or fluorescent materials aredescribed. Screens with phosphor materials under excitation of one ormore scanning excitation laser beams are described in detail and areused as specific implementation examples of optically excitedfluorescent materials in various system and device examples in thisapplication.

FIG. 1C illustrates an example of a laser-based display system using alight-emitting screen 101 under optical excitation of scanning beams120. In one implementation, for example, three different color phosphorsthat are optically excitable by the laser beam to respectively producelight in red, green, and blue colors suitable for forming color imagesmay be formed on the screen as pixel dots or repetitive red, green andblue phosphor stripes in parallel. Various examples described in thisapplication use screens with parallel color phosphor stripes foremitting light in red, green, and blue to illustrate various features ofthe laser-based displays.

Phosphor materials are one type of light-emitting materials. Variousdescribed systems, devices and features in the examples that usephosphors as the fluorescent materials are applicable to displays withscreens made of other optically excitable, light-emitting, non-phosphorfluorescent materials. For example, quantum dot materials emit lightunder proper optical excitation and thus can be used as the fluorescentmaterials for systems and devices in this application.

The system in FIG. 1C and other examples of scanning beam displaysystems based on light-emitting screens use at least one scanning laserbeam to excite color light-emitting materials deposited on a screen toproduce color images. The scanning laser beam is modulated to carryimages in red, green and blue colors or in other visible colors and iscontrolled in such a way that the laser beam excites the colorlight-emitting materials in red, green and blue colors with images inred, green and blue colors, respectively. Hence, the scanning laser beamcarries the images but does not directly produce the visible light seenby a viewer. Instead, the color light-emitting fluorescent materials onthe screen absorb the energy of the scanning laser beam and emit visiblelight in red, green and blue or other colors to generate actual colorimages seen by the viewer.

Laser excitation of the fluorescent materials using one or more laserbeams with energy sufficient to cause the fluorescent materials to emitlight or to luminesce is one of various forms of optical excitation. Inother implementations, the optical excitation may be generated by anon-laser light source that is sufficiently energetic to excite thefluorescent materials used in the screen. Examples of non-laserexcitation light sources include various light-emitting diodes (LEDs),light lamps and other light sources that produce light at a wavelengthor a spectral band to excite a fluorescent material that converts thelight of a higher energy into light of lower energy in the visiblerange. The excitation optical beam that excites a fluorescent materialon the screen can be at a frequency or in a spectral range that ishigher in frequency than the frequency of the emitted visible light bythe fluorescent material. Accordingly, the excitation optical beam maybe in the violet spectral range and the ultra violet (UV) spectralrange, e.g., wavelengths under 420 nm. In the examples described below,UV light or a UV laser beam is used as an example of the excitationlight for a phosphor material or other fluorescent material and may belight at other wavelength.

In FIG. 1C, the screen 101 is designed to have color phosphor stripes.Alternatively, color phosphor dots may also be used to define the imagepixels on the screen. The system includes a laser module 110 to produceand project at least one scanning laser beam 120 onto a screen 101. Thescreen 101 has parallel color phosphor stripes in the vertical directionand two adjacent phosphor stripes are made of different phosphormaterials that emit light in different colors. In the illustratedexample, red phosphor absorbs the laser light to emit light in red,green phosphor absorbs the laser light to emit light in green and bluephosphor absorbs the laser light to emit light in blue. Adjacent threecolor phosphor stripes are in three different colors. One particularspatial color sequence of the stripes is shown in FIG. 1C as red, greenand blue. Other color sequences may also be used. The laser beam 120 isat the wavelength within the optical absorption bandwidth of the colorphosphors and is usually at a wavelength shorter than the visible blueand the green and red colors for the color images. As an example, thecolor phosphors may be phosphors that absorb UV light in the spectralrange from about 380 nm to about 420 nm to produce desired red, greenand blue light. The laser module 110 can include one or more lasers suchas UV diode lasers to produce the beam 120, a beam scanning mechanism toscan the beam 120 horizontally from left to right and vertically fromtop to down to render one image frame at a time on the screen 101, and asignal modulation mechanism to modulate the beam 120 to carry theinformation for image channels for red, green and blue colors. Suchdisplay systems may be configured as rear scanning systems where theviewer and the laser module 110 are on the opposite sides of the screen101. Alternatively, such display systems may be configured as frontscanning systems where the viewer and laser module 110 are on the sameside of the screen 101.

FIG. 2A shows an exemplary design of the screen 101 in FIG. 1C. Thescreen 101 may include a rear substrate 201 which is transparent to thescanning laser beam 120 and faces the laser module 110 to receive thescanning laser beam 120. A second front substrate 202, is fixed relativeto the rear substrate 201 and faces the viewer in a rear scanningconfiguration. A color phosphor stripe layer 203 is placed between thesubstrates 201 and 202 and includes phosphor stripes. The color phosphorstripes for emitting red, green and blue colors are represented by “R”,“G” and “B,” respectively. The front substrate 202 is transparent to thered, green and blue colors emitted by the phosphor stripes. Thesubstrates 201 and 202 may be made of various materials, including glassor plastic thin or thick panels with various optical functions. Eachcolor pixel includes portions of three adjacent color phosphor stripesin the horizontal direction and its vertical dimension is defined by thebeam spread of the laser beam 120 in the vertical direction. As such,each color pixel includes three subpixels of three different colors(e.g., the red, green and blue). The laser module 110 scans the laserbeam 120 one horizontal line at a time, e.g., from left to right andfrom top to bottom to fill the screen 101. The laser module 110 is fixedin position relative to the screen 101 so that the scanning of the beam120 can be controlled in a predetermined manner to ensure properalignment between the laser beam 120 and each pixel position on thescreen 101.

In FIG. 2A, the scanning laser beam 120 is directed at the greenphosphor stripe within a pixel to produce green light for that pixel.FIG. 2B further shows the operation of the screen 101 in a view alongthe direction B-B perpendicular to the surface of the screen 101. Sinceeach color stripe is longitudinal in shape, the cross section of thebeam 120 may be shaped to be elongated along the direction of the stripeto maximize the fill factor of the beam within each color stripe for apixel. This may be achieved by using a beam shaping optical element inthe laser module 110. A laser source that is used to produce a scanninglaser beam that excites a phosphor material on the screen may be asingle mode laser or a multimode laser. The laser may also be a singlemode along the direction perpendicular to the elongated directionphosphor stripes to have a small beam spread that is confined by thewidth of each phosphor stripe. Along the elongated direction of thephosphor stripes, this laser beam may have multiple modes to spread overa larger area than the beam spread in the direction across the phosphorstripe. This use of a laser beam with a single mode in one direction tohave a small beam footprint on the screen and multiple modes in theperpendicular direction to have a larger footprint on the screen allowsthe beam to be shaped to fit the elongated color subpixel on the screenand to provide sufficient laser power in the beam via the multimodes toensure sufficient brightness of the screen.

Hence, the laser beam 120, which is modulated to carry optical pulseswith image data, needs to be aligned with respect to proper color pixelson the screen 101. The laser beam 120 is scanned spatially across thescreen 101 to hit different color pixels at different times.Accordingly, the modulated beam 120 carries the image signals for thered, green and blue colors for each pixel at different times and fordifferent pixels at different times. Hence, the beams 120 are coded withimage information for different pixels at different times. The beamscanning thus maps the timely coded image signals in the beams 120 ontothe spatial pixels on the screen 101.

A scanning display system described in this patent application can becalibrated during the manufacture process so that the laser beam on-offtiming and position of the laser beam relative to the fluorescentstripes in the screen 101 are known and are controlled within apermissible tolerance margin in order for the system to properly operatewith specified image quality. However, the screen 101 and components inthe laser module 101 of the system can change over time due to variousfactors, such as scanning device jitter, changes in temperature orhumidity, changes in orientation of the system relative to gravity,settling due to vibration, aging and others. Such changes can affect thepositioning of the laser source relative to the screen 101 over time andthus the factory-set alignment can be altered due to such changes.Notably, such changes can produce visible and, often undesirable,effects on the displayed images. For example, a laser pulse in thescanning excitation beam 120 may hit a subpixel that is adjacent to anintended target subpixel for that laser pulse due to a misalignment ofthe scanning beam 120 relative to the screen along the horizontalscanning direction. When this occurs, the coloring of the displayedimage is changed from the intended coloring of the image. Hence, a redflag in the intended image may be displayed as a green flag on thescreen. For another example, a laser pulse in the scanning excitationbeam 120 may hit both the intended target subpixel and an adjacentsubpixel next to the intended target subpixel due to a misalignment ofthe scanning beam 120 relative to the screen along the horizontalscanning direction. When this occurs, the coloring of the displayedimage is changed from the intended coloring of the image and the imageresolution deteriorates. The visible effects of these changes canincrease as the screen display resolution increases because a smallerpixel means a smaller tolerance for a change in position. In addition,as the size of the screen increases, the effect of a change that canaffect the alignment can be more pronounced because a large moment armassociated with a large screen means that an angular error can lead to alarge position error on the screen. For example, if the laser beamposition on the screen for a known beam angle changes over time, theresult is a color shift in the image. This effect can be noticeable andthus undesirable to the viewer.

A feedback control alignment mechanism can be provided in the system inFIG. 1C to maintain proper alignment of the scanning beam 120 on thedesired sub-pixel to achieved desired image quality. The screen 101 isused to provide a screen feedback signal 130 to indicate the alignmentstatus of the beam 120. When the alignment has an error, the controlmodule 110 responds to the error in the screen feedback to control thescanning beam 120 to compensate for the error. Such feedback control caninclude reference marks on the screen 101, both in the fluorescent areaand in one or more peripheral area outside the fluorescent area, toprovide feedback light that is caused by the excitation beam 120 andrepresents the position and other properties of the scanning beam on thescreen 101. The feedback light can be measured by using one or moreoptical servo sensors to produce a feedback servo signal. A servocontrol in the laser module 110 processes this feedback servo signal toextract the information on the beam positioning and other properties ofthe beam on the screen and, in response, adjust the direction and otherproperties of the scanning beam 120 to ensure the proper operation ofthe display system.

For example, a feedback servo control system can be provided to useperipheral servo reference marks positioned outside the display areaunobservable by the viewer to provide control over various beamproperties, such as the horizontal positioning along the horizontalscanning direction perpendicular to the fluorescent stripes, thevertical positioning along the longitudinal direction of the fluorescentstripes, the beam focusing on the screen for control the imagesharpness, and the beam power on the screen for control the imagebrightness. For another example, a screen calibration procedure can beperformed at the startup of the display system to measure the beamposition information as a calibration map so having the exact positionsof sub-pixels on the screen in the time domain. This calibration map isthen used by the laser module 110 to control the timing and positioningof the scanning beam 120 to achieve the desired color purity. For yetanother example, a dynamic servo control system can be provided toregularly update the calibration map during the normal operation of thedisplay system by using servo reference marks in the fluorescent area ofthe screen to provide the feedback light without affecting the viewingexperience of a viewer.

PCT Application No. PCT/US2007/004004 entitled “Servo-Assisted ScanningBeam Display Systems Using Fluorescent Screens” and filed on Feb. 15,2007 (PCT Publication No. WO 2007/095329) describes examples of feedbackcontrols for scanning beam systems suitable for use with 3D systemsdescribed in this application and is incorporated by reference as partof the specification of this application.

Referring now to FIG. 3, an example implementation of the laser module110 in FIG. 1C is illustrated. A laser array 310 with multiple lasers isused to generate multiple laser beams 312 to simultaneously scan thescreen 101 for enhanced display brightness. A signal modulationcontroller 320 is provided to control and modulate the lasers in thelaser array 310 so that the laser beams 312 are modulated to carry theimage to be displayed on the screen 101. The signal modulationcontroller 320 can include a digital image processor that generatesdigital image signals for the three different color channels and laserdriver circuits that produce laser control signals carrying the digitalimage signals. The laser control signals are then applied to modulatethe lasers, e.g., the currents for laser diodes, in the laser array 310.

The beam scanning in the system in FIG. 3 is achieved by using avertical adjuster 340 such as a galvo mirror for the vertical scanningand a 2-dimensional multi-facet polygon scanner 350 with differentfacets tilted at different angles. A scan lens 360 can be used toproject the scanning beams form the polygon scanner 350 onto the screen101. The scan lens 360 is designed to image each laser in the laserarray 310 onto the screen 101. Each of the different reflective facetsof the polygon scanner 350 simultaneously scans N horizontal lines whereN is the number of lasers. In the illustrated example, the laser beamsare first directed to the galvo vertical adjuster 340 and then from thegalvo vertical adjuster 340 to the polygon scanner 350 which scans thereceived laser beams as output scanning beams 120 onto the screen 101. Arelay optics module 330 is placed in the optical path of the laser beams312 to modify the spatial property of the laser beams 312 and to producea closely packed bundle of beams 332 for scanning by the polygon scanner350. The scanning beams 120 projected onto the screen 101 excite thephosphors and the optically excited phosphors emit colored light todisplay visible images.

The laser beams 120 are scanned spatially across the screen 101 to hitdifferent color pixels at different times. Accordingly, each of themodulated beams 120 carries the image signals for the red, green andblue colors for each pixel at different times and for different pixelsat different times. Hence, the beams 120 are coded with imageinformation for different pixels at different times by the signalmodulation controller 320. The beam scanning thus maps the time-domaincoded image signals in the beams 120 onto the spatial pixels on thescreen 101. For example, the modulated laser beams 120 can have eachcolor pixel time equally divided into three sequential time slots forthe three color subpixels for the three different color channels. Themodulation of the beams 120 may use pulse modulation techniques toproduce desired grey scales in each color, a proper color combination ineach pixel, and desired image brightness.

In one implementation, the optical relay module 330 can be an afocaldevice and includes a first lens having a first focal length to receiveand focus the laser beams from the lasers; a second lens having a secondfocal length shorter than the first focal length and spaced from thefirst lens by the first focal length to focus the laser beams from thefirst lens; and a third lens having a third focal length longer than thesecond focal length and spaced from the second lens by the third focallength to focus and direct the laser beams from the second lens to thescanning module. Examples for the afocal optical relay module 330 aredescribed in PCT application No. CT/US2006/041584 entitled “OpticalDesigns for Scanning Beam Display Systems Using Fluorescent Screens” andfiled on Oct. 25, 2006 (PCT publication No. WO 2007/050662) and U.S.patent application Ser. No. 11/510,495 entitled “Optical Designs forScanning Beam Display Systems Using Fluorescent Screens” and filed onAug. 24, 2006 (U.S. publication No. US 2007-0206258 A1), which areincorporated by reference as part of the specification of thisapplication.

In some implementations, an imaging module 370 can be placed in theoptical path between the vertical adjuster 340 and the polygon to imagethe surface of the reflective surface of the vertical adjuster 340 ontoa polygon facet that currently reflects the beams to the screen 101.This imaging effectively makes the vertical adjuster 340 coincident withthe currently reflecting polygon facet which, in turn, is coincidentwith the entrance pupil of the scan lens 360. Therefore, the entrancepupil of the scan lens 360 is the pivot point for the scanning beamsdirected to the scan lens 360. The imaging module 370 can be in variousoptical configurations and may include, for example, two lenses in a 4 fimaging configuration with a magnification of 1.

FIG. 4A shows an example of a two-dimensional polygon scanner 400 for ascanning beam display system, such as the systems in FIGS. 1A, 1C and 3.The polygon scanner 400 has a rotation axis represented by the line 401along the vertical direction and has multiple reflective polygon facets(e.g., 410, 420, 430, 440, 450, 460 and 470) that are tilted atdifferent tilt facet angles with respect to the rotation axis 401.

FIG. 4B shows the vertical stepping by different polygon facets of thepolygon scanner 400 in FIG. 4A. A vertical adjuster 480 directs light490 to the polygon scanner 400 that rotates around its vertical rotationaxis 401. In operation, the polygon scanner 400 rotates at a constantspeed around the vertical rotation axis 401 and the incident opticalbeam at a fixed incident direction from the vertical adjuster 480 isreflected by different facets at different angles in the verticaldirection as the horizontal scanning beams 12 towards the screen 1.

In operation, the light of the scanning beams 12 is turned off incertain times to create blanking periods so that no light is projectedonto the screen 1 in order to minimize undesired visual effects on thescreen 1. For example, during the rotation of the polygon scanner 400,each beam incident to the polygon scanner 400 is turned off for a shortperiod when the boundary between two adjacent facets scans through theincident beam to avoid a portion of the beam less than the whole beam isdirected to the screen 1. Referring to FIG. 4A, the polygon facet 410 ismarked to show a central scanning region 411 for reflecting and scanninga beam onto the screen and two blanking regions 412 and 413 on two sidesof the central scanning region 411 that with a width less than a beamdiameter from the divider of two adjacent facets. The light of each beamis turned off when the beam falls in a blanking region 412 or 413 withonly a portion of the whole beam spot being on the facet 410. Thisblanking reduces unwanted scattered light on to the screen 1 and improvethe image quality.

In addition to the above transition from one facet to the subsequentfacet, the system also experiences a transition phase between the end ofone full rotation of the polygon scanner 400 and the beginning of thesubsequent full rotation of the polygon scanner 400. During scanningwithin one full rotation of the polygon for producing horizontal linesfor one field of two interlaced fields for a full frame, the verticaladjuster is fixed at a given vertical position. The vertical adjusterchanges its orientation to a different fixed orientation aftercompletion of one full rotation and before the next full rotation. Thistransition requires another blanking time during which the light in eachoptical beam is turned off to reduce undesired visual effects on thescreen.

FIG. 5 illustrates the parallel horizontal scan lines formed on a screenfor the system in FIGS. 4A and 4B and shows the blanking periods whenlight to the screen is turned off. Two different types of blanking timesare shown. The first type is the blanking time for the vertical steppingby the polygon when transitioning across the boundary regions of twoadjacent facets during a horizontal scanning by the polygon scanner. Thesecond type is the blanking time when the vertical adjuster adjusts itsorientation to shift the vertical position of an image field withrespect to a preceding imaging field to interlace the two image fieldsto form a full image.

The blanking time during transitioning between two adjacent facets andthe blanking time during transitioning between two sequential fullrotations can be implemented in various configurations. Referring toFIG. 4A, for example, one way for providing the blanking times is todesign all polygon facets identical in size and shape and with adimension along the horizontal scanning direction longer than thescanning region 411 to have blanking regions 412 and 413 at both ends ofeach facet. Under this design, the light of each beam is turned off whenthe beam is located in the blanking region 412 or 413. The blankingregions 412 and 413 are designed to be sufficiently long to accommodatefor the desired blanking times for transitioning between adjacent facetsand between two sequential full rotations of the polygon scanner 400.

As another example, the polygon facets can be made identical and onefacet is designated as a blanking facet where the light of each beam isturned off during the time the beam falls on the blanking facet tofacilitate the transition between two sequential full rotations of thepolygon scanner 400.

FIG. 6A shows another example of a polygon scanner 600 where a special“blanking” polygon facet 601 is provided and is configured to have adifferent size from regular polygon facets. In many applications, thisblanking polygon facet 601 is made to be smaller than the regularpolygon facets. In operation, the light of each beam is turned offduring the time the beam falls on the blanking facet 601 to facilitatethe transition between two sequential full rotations of the polygonscanner 600.

FIG. 6B shows yet another example of a polygon scanner where one facetis selected to have an extended region that is used to provide a special“blanking” region. The light of each beam is turned off during the timethe beam falls on the blanking region of the polygon facet while thelight is turned on when the beam is at other portion of the same polygonfacet. This extended blanking region is to facilitate the transitionbetween two sequential full rotations of the polygon scanner.

Referring back to the example in FIG. 3, the vertical adjuster 340 ispositioned upstream from the polygon scanner 350 (which is know the twodimensional polygon scanner) to direct the optical beams from the relayoptics module 330 to the polygon scanner 350 which in turn scans thebeams in two dimension onto the screen 101. Under this optical traindesign, the optical path for each optical beam 120 is folded and thevertical adjuster can be located closer to the screen 101 than thepolygon scanner 350 under various arrangements.

As an example, the two dimensional polygon scanner 350 and the verticaladjuster340 can be placed at two locations that are horizontally at acenter of the screen 101 and the vertical adjuster 340 is closer to thescreen 101 than the polygon scanner 350 to direct the optical beams 120to the polygon scanner 350 at an acute angle with respect to therotation axis of the polygon scanner 350. The screen 101 may be tiltedwith respect to the he rotation axis of the polygon scanner 350 toreduce a symmetric image distortion on the screen 101. The verticaladjuster340 can be located below or above the vertical position of thepolygon scanner 350.

FIG. 7A shows an example where the vertical adjuster 340 is placedbetween the screen 101 and the polygon scanner 350 at a position belowthe polygon scanner 350 along the vertical direction with a 45-degreeangle of incidence with respect to the vertical direction. FIG. 7B showthe border trace of the projected image area on the screen which isdistorted.

FIG. 8A shows tilting the screen in the system in FIG. 7A towards thepolygon scanner by 1.25 degrees to reduce the image distortion. FIG. 8Bshows the border trace of the projected image area on the tilted screenin which the distortions are reduced in within the screen area.

As another example, the polygon scanner 350 and the vertical adjuster340in FIG. 3 can be placed at two locations that are horizontally offsetfrom each other and the vertical adjuster340 is closer to the screen 101than the polygon scanner 350 to direct the optical beams 120 to thepolygon scanner 350. The vertical adjuster340 and the polygon scanner350 can be at the same height to direct the incident beam from thevertical adjuster340 to the polygon scanner 350 at a side feedarrangement.

FIG. 9A shows an example of the side feed arrangement with a 45-degreeangle of incidence from the vertical adjuster to the polygon scanner inthe same horizontal plane. FIG. 9B shows the border trace of theprojected image area on the screen in which the distortions are present.FIG. 10A shows tilting the screen around a vertical axis to counter thedistortion caused by the side feed arrangement to reduce the distortions(FIG. 10B).

In the above examples, the vertical adjuster is used to hold thevertical position of each beam at a fixed vertical position on thescreen during a horizontal scan by the polygon scanner and to adjust,during a blanking period, vertical positions of parallel horizontallines in one group relative to vertical positions of parallel horizontallines in another group produced subsequent to the prior group on thescreen. This vertical adjustment can be used to interlace two or moreimage fields to form a full frame with the number of horizontal linesequal to the sum of the interlaced image fields.

Interlacing two image fields is illustrated in the example in FIG. 1B.The spacing between two adjacent lines on the screen produced byreflection of beams from a single polygon facet can be set to (P-1)where P is the number of fields to be interlaced and is an integer notless than 2. Hence, the spacing between the scanning lines on the screenformed by two adjacent laser beams reflected from a one polygon facetcan be one horizontal line for interlacing two fields and two horizontallines for interlacing three fields.

In addition, the vertical adjuster can be used to stack two or moredifferent image fields along the vertical stepping direction to form afull image. The control unit is configured to control the verticaladjuster to scan the optical beams over a first surface segment at afirst fixed position of the vertical adjuster in a full rotation of thepolygon scanner and to scan the optical beams over a second surfacesegment that is vertically displaced from and does not overlap with thefirst surface segment when the vertical adjuster is at a second fixedposition in a subsequent full rotation of the polygon scanner.

FIG. 11 shows an example of this operation mode of the verticaladjuster. In this example, one full rotation of the polygon producesField 1 with N×M parallel horizontal lines as shown. Next, the verticaladjuster is operated during a blanking time before the next scanning forthe field 2 to move the vertical positions of the beams to produce thefield 2 below the field 1. At the end of this blanking time, the lightof the beams is turned on to allow the polygon scanner to project N×Mparallel horizontal lines for the field 2. This operation allows animage of 2×N×M horizontal lines to be formed on the screen.

In actual implementations of the 2-dimensional scanning polygondescribed here, each facet may deviate from a desired tilt angle bydesign due to imprecision in manufacturing and other factors. Thisdeviation of the tilt facet angle is an error and can cause errors invertical positions of different horizontal lines scanned by differentfacets of the polygon scanner. This facet angle error can degrade theimage quality on the screen.

A polygon scanner can be designed and manufactured with a high precisionto minimize the facet angle error. Polygons with low facet angle errors,however, can be expensive. To reduce the cost, a facet angle errorcorrection mechanism can be implemented in such a system to correct theknown facet angle errors of an installed polygon scanner. Implementationof this correction mechanism allows the use of relatively inexpensivepolygons with facet angle errors without compromising the displayperformance. In addition, the orientations of facets of a polygonscanner may change with time due to various factors, such as a change intemperature and other environmental factors (e.g., humidity), aging ofthe materials used in a polygon scanner over time, and others.Furthermore, a polygon scanner in a system may be replaced by adifferent polygon scanner due to malfunction or failure of the originalpolygon and such replacement can change the facet angle errors becausetwo different polygons tend to have different facet angle errors. Hence,to maintain a high image quality in presence of variations of facetangle errors, the facet angle error correction mechanism can be designedto provide adjustable corrections to the facet angle errors tocounteract to different errors associated with different facet anglesand variations of the facet angle errors of facets.

For example, the vertical adjuster can be used to make an adjustment toits vertical orientation based on a known facet angle error for aspecific polygon facet to correct the effect of the known error. Thefacet angle errors can be measured and stored in a look-up table. When afacet angle error does not change significantly with temperature,humidity and others, this look-up table method may be sufficient withoutusing the servo feedback based on a measured vertical beam positionusing the vertical reference mark described above. In implementation,the feedback control needs the identification of the polygon facet thatis currently scanning a line and thus can retrieve the correspondingfacet angle error value for that polygon facet from the look-up table.The identification of the current polygon facet can be determined from afacet number sensor on the polygon.

In operation, the above facet angle correction based on adjusting thevertical adjuster is activated and applied during the light-off periodwhen transitioning from one facet to a subsequent facet. The scanningcontrol module in the system uses the facet identification number of thesubsequent facet to look up and retrieve the facet angle error from thestored look-up table and applies the a correction to the verticalorientation of the vertical scanner to counter the retrieved error.After this correction, the scanning with the subsequent facet begins.This process is executed at every light-off period.

The vertical adjuster may be operated to simply provide the above facetangle correction based on adjusting the vertical adjuster during thelight-off period when transitioning from one facet to a subsequentfacet, without performing the interlacing and the stacking differentfields of images. Under this design, the full frame of the 2-dimensionalscanning by the polygon has N×M horizontal scanning lines.

The above techniques and designs for using a 2D polygon scanner and avertical adjuster to can be implemented in various scanning beam systemsother than display systems as described in the above examples. Forexample, the present scanning systems based on two-dimensional polygonscanner can be used in optical systems that use scanning beams toproduce optical patterns. For example, laser printing systems can usethe present scanning systems where the screen is replaced by a printingmedium (e.g., paper, fabric, or a master printing plate). The printingmedium is used to receive the one or more scanning laser beams thatcarry the images to be formed on the print medium. The images carried bythe light can be formed on the printing medium based various photoeffects, e.g., photomechanical, photochemical, or laser engravingprocesses. The printing medium can be a printing plate which is thenused to transfer the images onto paper or other printing materials. Thelasers in the laser array can be implemented by lasers such as diodelasers that emit at a proper wavelength for the laser printing operationon the printing medium. One specific example is printing systems thatproduce mask patterns based on laser scanning where rapid turns aredesired. This printing can eliminate the need for mask preparation.

While this patent application contains many specifics, these should notbe construed as limitations on the scope of an invention or of what maybe claimed, but rather as descriptions of features specific toparticular embodiments of the invention. Certain features that aredescribed in this patent application in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or a variation of a subcombination.

Only a few implementations are disclosed. However, it is understood thatvariations, enhancements and other implementations can be made based onwhat is described and illustrated in this patent application.

1. A scanning beam system, comprising: an optical module operable toproduce a plurality of optical beams modulated to carry images to scanover a surface to produce the images on the surface based on a twodimensional scanning pattern, wherein the optical module comprises: apolygon scanner positioned in optical paths of the optical beams andcomprising a rotation axis around which the polygon scanner rotates toscan the optical beams horizontally on the surface, a plurality ofpolygon facets that are sized to simultaneously receive the opticalbeams and comprise a plurality of reflective polygon facets are tiltedwith respect to the rotation axis at different facet tilt angles,respectively, to scan the optical beams horizontally at differentvertical positions on the surface, respectively; a vertical adjusterplaced in the optical paths of the optical beams to control and adjustvertical positions of the optical beams on the surface; and a controlunit that controls the vertical adjuster at a fixed position to placethe optical beams at respective fixed vertical positions on the surfacewhen the polygons scanner horizontally scans the optical beams toproduce parallel horizontal lines on the surface.
 2. The system as inclaim 1, wherein: the control unit is configured to turn off light ofthe optical beams during a blanking period when positions of the opticalbeams are near and at the border between two polygon facets, wherein theblanking period corresponds to a duration when each optical beam ispositioned within a zone near the edge of the facet by a dimension lessthan a beam width of each optical beam along the horizontal direction.3. The system as in claim 1, comprising: a screen which provides thesurface on which the images are displayed and comprises parallelfluorescent stripes substantially perpendicular to the horizontaldirection to absorb light of the optical beams to emit visible lightwhich produces the images.
 4. The system as in claim 1, wherein: the twodimensional scanning pattern is an interlaced pattern to form a fullframe display by interlacing two or more different patterns ofhorizontal lines produced by two or more different sequential fullrotations of the polygon scanner at different vertical positions of thevertical adjuster, respectively.
 5. The system as in claim 4, wherein:the control unit is configured to turn off light of the optical beamsfor a blanking period equal to a time duration for the vertical adjusterto adjust a vertical angle to switch from projecting one pattern ofhorizontal lines to projecting another interlaced pattern of horizontallines.
 6. The system as in claim 1, wherein: the optical module controlsa vertical angle of the vertical adjuster when a reflective polygonfacet scans the optical beams to reduce an error in vertical positionsof the optical beams on the surface caused by a deviation of the facettilt angle of the reflective polygon facet from a desired facet tiltangle designed for the reflective polygon facet.
 7. The system as inclaim 1, wherein: the vertical adjuster is positioned upstream from thepolygon scanner to direct the optical beams to the polygon scanner. 8.The system as in claim 7, wherein: the polygon scanner and the verticaladjuster are placed at two locations that are horizontally at a centerof the surface and the vertical adjuster is closer to the surface thanthe polygon scanner to direct the optical beams to the polygon scannerat an acute angle with respect to the rotation axis of the polygonscanner.
 9. The system as in claim 8, wherein: the surface is tiltedwith respect to the rotation axis of the polygon scanner.
 10. The systemas in claim 1, wherein: the control unit is configured to control thevertical adjuster to scan the optical beams over a first surface segmentat a first fixed position of the vertical adjuster in a full rotation ofthe polygon scanner and to scan the optical beams over a second surfacesegment that is vertically displaced from and does not overlap with thefirst surface segment when the vertical adjuster is at a second fixedposition in a subsequent full rotation of the polygon scanner.
 11. Ascanning beam system, comprising: an optical module operable to producea plurality of optical beams modulated to carry images to scan over asurface to produce the images on the surface, wherein the optical modulecomprises: a polygon scanner positioned in optical paths of the opticalbeams and comprising a rotation axis around which the polygon scannerrotates to scan the optical beams horizontally on the surface, and aplurality of polygon facets that are sized to simultaneously receive theoptical beams and reflective to light of the optical beams, the polygonfacets tilted with respect to the rotation axis at different tiltangles, respectively, to scan the optical beams horizontally atdifferent vertical positions on the surface, respectively, a verticaladjuster placed in the optical paths of the optical beams and to controland adjust vertical positions of the optical beams on the surface, and ascanning control mechanism to synchronize the vertical adjuster to thepolygon scanner to adjust vertical positions of the optical beams on thesurface to spatially interlace one frame of a sequential sets ofsimultaneous horizontal scanning lines on the surface produced by thepolygon facets, respectively, one set per polygon facet, in one fullrotation of the polygon scanner with a subsequent frame of a sequentialsets of simultaneous horizontal scanning lines on the surface producedby the polygon facets, respectively, one set per polygon facet, in animmediate subsequent full rotation of the polygon scanner.
 12. Thesystem as in claim 11, comprising: a screen which provides the surfaceon which the image are displayed and comprises parallel fluorescentstripes substantially perpendicular to the horizontal direction toabsorb light of the optical beams to emit visible light which producesthe images.
 13. The system as in claim 11, wherein: at least onereflective polygon facet of the polygon scanner has a known deviation inthe facet tilt angle from a desired facet tilt angle; and the scanningcontrol mechanism controls a vertical angle of the vertical adjusterwhen the reflective polygon facet with the known deviation in the facettilt angle scans the optical beams to offset a vertical angle of each ofthe optical beams for reducing an error in vertical positions of theoptical beams on the surface caused by the known deviation of thereflective polygon facet.
 14. The system as in claim 11, wherein: thevertical adjuster is positioned upstream from the polygon scanner todirect the optical beams to the polygon scanner.
 15. The system as inclaim 14, wherein: the polygon scanner and the vertical adjuster areplaced at two locations that are horizontally at a center of the surfaceand the vertical scanner is closer to the surface than the polygonscanner to direct the optical beams to the polygon scanner at an acuteangle with respect to the rotation axis of the polygon scanner.
 16. Thesystem as in claim 15, wherein: the surface is tilted with respect tothe he rotation axis of the polygon scanner.
 17. A method for scanningoptical beams in a scanning beam system, comprising: producing aplurality of optical beams modulated to carry images to scan over asurface to produce the images on the surface, using a polygon scanner inoptical paths of the optical beams, which comprises a rotation axisaround which the polygon scanner rotates and a plurality of polygonfacets that are sized to simultaneously receive the optical beams andreflective to light of the optical beams, to scan the optical beamshorizontally on the surface, wherein the polygon facets are tilted withrespect to the rotation axis at different tilt angles, respectively, toscan the optical beams horizontally at different vertical positions onthe surface, respectively, to produce one frame of a sequential sets ofsimultaneous horizontal scanning lines on the surface produced by thepolygon facets, respectively, one set per polygon facet, in one fullrotation of the polygon scanner; and holding a vertical position of eachoptical beam on at a fixed position when each optical beam is beinghorizontally scanned on the surface and the light of each optical beamis projected onto the surface, without scanning each optical beam alongthe vertical direction.
 18. The method as in claim 17, comprising: usinga vertical adjuster in the optical paths of the optical beams to controlvertical positions of the optical beams on the surface and to adjustvertical positions of the optical beams on the surface to spatiallyshift horizontal scanning lines in one frame produced in one fullrotation of the polygon scanner with horizontal scanning lines of asubsequent frame produced in an immediate subsequent full rotation ofthe polygon scanner.
 19. The method as in claim 17, comprising:controlling a vertical angle of the vertical adjuster when a reflectivepolygon facet scans the optical beams to reduce an error in verticalpositions of the optical beams on the surface caused by a deviation ofthe tilt angle of the reflective polygon facet from a desired facet tiltangle designed for the reflective polygon facet.
 20. The method as inclaim 17, comprising: using a vertical adjuster in the optical paths ofthe optical beams to control vertical positions of the optical beams onthe surface; and controlling the optical beams to direct no light of theoptical beams to the surface during a time when the vertical scanneradjusts its vertical position between one full rotation of the polygonscanner and an immediate subsequent full rotation of the polygonscanner.